Integrated morphological, metabolomic, and transcriptomic analysis unravels the mechanism of floral blooming in Lonicera japonica cultivars

Honeysuckle ( Lonicera japonica ), valued for the heat-clearing and detoxifying properties of its unopened flower buds, is widely used as a bulk medicinal herb in healthcare. While the regulatory mechanisms underlying flower opening remain poorly understood. To address it, present study analyzed two cultivars, Baijin 1 and Baijin 2, which exhibited distinct corolla morphologies. First, measurements of flower fresh weight, corolla area, and active component content across developmental stages indicated that Baijin 2 possesses a notably longer harvesting window. Building on morphological assessments, cross-sectional and microscopic analyses further revealed that Baijin 1 exhibits a greater number of external vascular bundles, pronounced elongation and expansion of corolla epidermal cells with enlarged internal vacuoles. Additionally, targeted metabolomics demonstrated substantially higher levels of total plant hormones, glucose, and sucrose in Baijin 1 during corolla expansion, whereas Baijin 2 accumulated more raffinose and trehalose. Subsequent transcriptomic profiling showed that differentially expressed genes (DEGs) in Baijin 1 were predominantly enriched in the pentose and glucuronate interconversions pathway, with key genes encoding polygalacturonase and pectate lyase-like proteins being significantly upregulated. In contrast, DEGs in Baijin 2 were largely associated with phytohormone biosynthesis and signaling pathways. Based on these findings, we propose a flowering mechanism in which mechanical tension from vascular bundles, turgor pressure generated through sugar conversion in epidermal cells, and gene-modulated phytohormone signaling act coordinately to drive corolla expansion. In conclusion, this study advances our understanding of key agronomic traits in honeysuckle, and also provides a valuable model for investigating floral opening mechanisms.